Intravascular ultrasound systems allow clinicians to image within coronary arteries and other vessels. Typically, the clinician will route a guidewire from the femoral artery, through the aorta and into the coronary arteries. When satisfied with the location of the guidewire, the clinician guides an ultrasound catheter having a transmission line coupled to a rotating transducer at its distal end along the guidewire to the coronary arteries. As illustrated in FIG. 1, signals produced by the transduce 10 must travel through the transmission line 15 before being processed in the signal processor 20. Because the signals are of small amplitude and must travel through 5 to 6 feet of transmission line before being processed, noise received during transmission in the transmission line presents a serious problem.
In addition to the noise added by the transmission line, intravascular ultrasound systems face other problems. For example, because the transducer typically comprises a resonant piezoelectric structure, the impedance of the transducer has a significant capacitive component. The efficiency and bandwidth of the transducer are raised if the impedance of the transducer is "tuned" by adding, for example, an inductive component to cancel the imaginary component of the transducer impedance. A wider bandwidth enables a shorter ultrasound pulse width, thus improving image resolution. In addition, the transducer should be impedance matched to the characteristic impedance of the transmission line for maximum performance.
There is a need in the art for improved intravascular ultrasound systems which reduce the effect of noise added from the transmission line and which ease the tuning and matching of the transducer to the transmission line. A preamplifier located adjacent to the rotating transducer and distally to the transmission line increases the amplitude of the transducer signal, lessening the effect of noise. Moreover, the use of such a preamplifier reduces the need for tuning and eases matching of the transducer impedance. Because a preamplifier located adjacent to the transducer will be exposed to high amplitude electric pulses used to excite the transducer, such a preamplifier should be protected with circuitry which prevents the preamplifier from being damaged from these excitations.
Prior art ultrasound systems with rotatable transducers have failed to address the problem caused by the transmission line noise. For example, Andou et al., U.S. Pat. No. 4,674,515 disclose an endoscopic ultrasound system with a rotatable transducer located at the distal end of a transmission line. Although the system disclosed by Andou et al. possesses a preamplifier, this preamplifier is located at the proximal end of the cable so that it cannot improve the signal-to-noise ratio before signal transmission through the transmission line. In addition, a proximally located preamplifier does not simplify tuning or matching of the transducer impedance.